Using mass air flow to provide a control input to an electronic engine control system ("ECS") is well known, as is evidenced by the following listing of exemplary prior issued commonly-assigned U.S. patents generally relating to electronic engine control arrangements:
U.S. Pat. No. 4,421,089 PA1 U.S. Pat. No. 4,401,063 PA1 U.S. Pat. No. 4,387,602 PA1 U.S. Pat. No. 4,250,842 PA1 U.S. Pat. No. 4,246,639 PA1 U.S. Pat. No. 4,245,317 PA1 U.S. Pat. No. 4,228,777 PA1 U S. Pat. No. 4,214,307 PA1 U S. Pat. No. 4,212,065 PA1 U S. Pat. No. 4,193,380 PA1 U S. Pat. No. 4,186,602 PA1 U S. Pat. No. 4,096,833 PA1 U S. Pat. No. 4,096,831 PA1 U S. Pat. No. 4,091,773. PA1 U.S. Pat. No. 4,860,222 PA1 U.S. Pat. No. 4,587,884 PA1 U.S. Pat. No. 4,448,070 PA1 U.S. Pat. No. 4,494,405 PA1 U.S. Pat. No. 4,445,369 PA1 U.S. Pat. No. 4,433,576 PA1 U.S. Pat. No. 4,125,093 PA1 U.S. Pat. No. 4,425,886 PA1 U.S. Pat. No. 4,403,506 PA1 U.S. Pat. No. 4,317,365 PA1 U.S. Pat. No. 4,237,830 PA1 U.S. Pat. No. 4,083,244 PA1 U.S. Pat. No. 3,433,069 PA1 U.S. Pat. No. 3,374,673
The following additional patents also relate to air flow sensing arrangements within electronic engine control systems:
As is well known, mass air flow parameters are extremely useful in controlling the operation of an internal combustion engine. Mass air flow relates to the mass of the air actually flowing through (into) the engine, and thus provides information useful in calculating and controlling critical engine operating parameters such as air-fuel ratio. One of the advantages of measuring mass air flow directly using, for example, a mass air flow meter (as opposed to indirectly using an air volume flow meter) is independence of the measurement on variables such as engine tolerances (which may change as the engine wears). See, for example, Loesing et al, "Mass Air Flow Meter--Design and Application", SAE Technical Paper Series No. 890779 (International Congress and Exposition, Detroit, Mich., Feb. 27-Mar. 3 1989).
Typical conventional mass air flow meters found in many of today's automotive systems operate on the hot wire anemometer principle. Briefly, a hot film or wire is heated by an electrical current so as to maintain a constant temperature differential between the heated element and another non-heated (i.e., at ambient temperature) element. The air flowing past the heated element removes heat from that element (with higher mass air flow removing more heat)--requiring additional electrical heating current to maintain the heated element at the constant temperature differential above ambient. A voltage differential V.sub.out appearing across a resistor coupled (typically in series) with the heating element is measured or otherwise used to provide a direct measure of mass air flow.
As is well known, this hot wire anemometer type mass air flow sensor provides mass air flow as a fourth order function of the voltage output signal: ##EQU1## where V.sub.out is the output voltage, k.sub.1 and k.sub.2 are constants, and m.sub.L is the mass air flow. FIG. 1 shows a typical transfer function for an exemplary hot wire anemometer type mass air flow sensor showing this fourth-order relationship.
One problem arises as to how to efficiently obtain the mass air flow from the sensor V.sub.out output without introducing errors or using costly components.
In automotive fuel management systems, it is desirable to calculate or estimate the mass of air taken into a corresponding individual combustion chamber cylinder during the intake stroke (in a Otto cycle type four-stroke internal combustion engine for example) in order to determine the amount of fuel that must be injected into that cylinder so as to provide a desired air/fuel ratio. Unfortunately, the air flow is anything but constant over an engine cycle, but rather may be more accurately thought of as surges or pulses of air flowing into the cylinder during the time the intake valve is open.
One technique used in the past to determine the air mass flowing into a cylinder during the intake stroke is to apply a wave form factor to the sampled air flow value. However, this technique is generally successful only if the wave shape is constant and the sample location on the wave form is known. Neither of these conditions exist in modern engines including variable valve timing. Variable valve timing control can add large variations to the mass air flow during an engine cycle. The wave shapes of these variations are not predictable, and wave shape factor and/or synchronous sampling techniques are therefore not effective to provide accurate mass air flow determinations based on more limited measurement information. To obtain the air "charge" (trapped air mass) in a cylinder combustion chamber under these circumstances, the air flow may be integrated (e.g., with respect to time) for each cylinder "event" (e.g., intake stroke) using a sufficiently large number of sufficiently high resolution samples to yield an accurate air mass determination.
One attempt to filter (integrate) the flow signal using analog circuitry introduced large errors attributable to the non-linear nature of the V.sub.out signal. Difficulties with this method can be demonstrated by providing a somewhat simplified but nevertheless illustrative example. The following Table I provides the transform for a typical mass air flow sensor:
TABLE I ______________________________________ Sensor Calibration Mass Air Flow kg/hr Sensor Voltage X Y ______________________________________ 13.0000 0.48500 15.00000 0.55500 20.00000 0.71700 25.00000 0.84200 30.00000 0.94000 35.00000 1.02400 40.00000 1.10400 45.00000 1.18200 50.00000 1.25600 60.00000 1.39400 70.00000 1.52000 80.00000 1.63500 90.00000 1.74100 100.00000 1.83900 110.00000 1.92900 130.00000 2.09400 150.00000 2.24000 160.00000 2.30800 170.00000 2.37200 180.00000 2.43500 190.00000 2.49400 200.00000 2.55300 210.00000 2.60900 230.00000 2.71500 250.00000 2.81200 270.00000 2.90900 300.00000 3.04400 350.00000 3.25200 400.00000 3.44200 450.00000 3.61500 500.00000 3.77600 550.00000 3.92900 600.00000 4.07300 650.00000 4.20900 700.00000 4.33600 750.00000 4.45800 850.00000 4.69200 ______________________________________
The left-hand (X) column sets forth mass air flow in kg/hour, and the right-hand (Y) column indicates the corresponding mass air flow sensor output voltage V.sub.out (in volts) for an exemplary mass air flow sensor.
Assume for purposes of this example a simplified combustion cylinder air intake waveform in which the flow is 600 kg/hr for 1/2 second and then drops to 30 kg/hr for 1/2 second. The correct average (integrated) value of mass air flow during the one second sample period would then be given by: EQU (600 kg/hr * 0.5 seconds)+(30 kg/hr * 0.5 seconds) =315 kg/hr * sec.
If the corresponding voltages V.sub.out from Table I are referenced, it will be seen that 600 kg/hr corresponds to an output voltage V.sub.out of 4.073 V, and 30 kg/hr corresponds to a sensor output voltage V.sub.out of 0.940 V.
Integrating these voltages V.sub.out over time yields the following result: EQU (4.073 V * 0.5 sec)+(0.940 V * 0.5 sec) =2.507 volts sec.
From Table I (using interpolation), 2.506 volts corresponds to only 193 kg/hr- This represents an error of 39% with respect to the actual value of 313 kg/hr. FIG. 2 shows these values superimposed on the exemplary sensor transfer function curve shown in FIG. 1. The error arises because of the non-linear nature of the V.sub.out signal. Accordingly, it is desirable to linearize the signal so as to eliminate non-linearity.
It is generally known to linearize a non-linear analog signal by converting the non-linear signal to a digital value and to then map or convert (e.g., using a look-up table stored in a read only memory device) the resulting digital value into a linearized value. Unfortunately, when the V.sub.out signal from a mass air flow sensor is converted to the digital domain for subsequent digital processing, special attention to accuracy of the lower end of the scale is required to obtain adequate resolution due to the fourth-order characteristic of the sensor transfer function (see the "crowding" of points on the portions of the FIG. 1 curves corresponding to flows less than 250 kg/hour, for example). Thus, a high cost, higher resolution digital-to-analog (D/A) converter is typically required to obtain the resolution required (even though the higher resolution is really only required on the lower end of the scale).
The present invention provides an improved electronic internal combustion engine control system and technique which more effectively utilize measured mass air flow.
In accordance with one feature of the invention, a lower cost non-linear A/D converter (e.g., of the type commonly used in the communications field) can be used to convert an analog output signal produced by a mass air flow sensor into a digital signal. Such A/D converters offer higher resolution at lower cost, but also introduce further non-linearities into an already non-linear signal. In accordance with this feature of the present invention, the digital output of the A/D converter is applied to a look-up table (e.g., implemented by a ROM storing predetermined mapping information) containing linearizing information at each memory location. The linearizing information is defined by the combined functions of: (a) conversion to linearize the output of the mass air flow sensor, and (b) further conversion to eliminate the non-linearities introduced by the non-linear A/D converter. Thus, the two functions required to obtain a linear digital signal can be combined into a single look-up table--saving resources (memory and time) in the processing of the digital signal.
An electronic circuit thus accepts a non-linear analog signal from a Mass Air Flow (MAF) sensor and converts it to a digital signal by means of a linear or non-linear analog to digital (A/D) converter. The digital signal may then be processed by a two-dimensional look-up table which includes corrections for the MAF sensor non-linearity and additional corrections for non-linearity of the A/D converter. This linearized MAF signal is then integrated or averaged to provide air mass per engine event or average mass air flow during an event. This circuit is useful in obtaining better accuracy in fuel management systems that use MAF sensors, particularly if variable valve control is included as part of the control system. The look-up table and integrator can be easily implemented in a digital signal processor or microcontroller.
Digital linearized flow is thus integrated in the preferred embodiment of the present invention by summing samples taken at regular fixed engine positions, and by dividing by the number of samples to obtain average flow over the cycle. Alternatively, the sample may be multiplied by the time between samples and summed to obtain total air mass for the cycle. If total mass per cycle is desired and the sampling rate is constant rate (not fixed engine degrees), then the samples can be summed over the cycle and the resulting sum may be multiplied by the fixed sampling time (thereby saving a multiplication per sample).
The present invention also provides an improved technique for accurately controlling various functions of an internal combustion engine using electronically measured mass air flow into the engine. A microprocessor based electronic control unit (ECU) may receive input signals from a mass air flow sensor (MAFS) as well as other engine operating parameters (e.g., engine speed or period, coolant temperature, throttle position, etc.). An engine load factor (representing the trapped mass of air in a given combustion chamber) is calculated by multiplying the MAFS signal with the engine period. This load factor better utilizes the MAFS output--giving a result similar to the commonly used manifold pressure without the need for such a special sensor. The load factor may then be used to program spark timing, correct the fuel pulse width, program idle/deceleration air, program acceleration enrichment/deceleration enleanment and deceleration fuel cutoff, bias closed loop control, and/or other control functions using conventional engine control algorithms.